This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
DL downlink (eNB to UE)
eNB EUTRAN Node B (evolved Node B/base station)
E-UTRAN evolved UTRAN (LTE)
LTE long term evolution
PCI physical cell indicator/indication
RF radio frequency
RNC radio network controller (UTRAN)
UE user equipment
UL uplink (UE to eNB)
UTRAN universal terrestrial radio access network
Recent progression in the wireless arts include the deployment of smaller cells within the coverage areas (or extending the coverage area) of conventional cells. In at least the LTE system these smaller cells are generally termed pico or nano cells while the conventional cells are distinguished by the term macro cell, with the aggregated deployment referred to as a heterogeneous network or HetNet for short. An exemplary HetNet is shown at FIG. 1, in which there are pico eNBs 16, 18 with control over relatively small cells B, C are within the coverage area of a macro cell A controlled by a macro eNB 12. It is conventional to use the terms cell and eNB or access node interchangeably, so for example the UE 10 with the macro cell 12 as its serving cell will as shown in FIG. 1 see cells 16 and 18 as neighbor cells. In the UTRAN and E-UTRAN the serving cell broadcasts in its system information a neighbor cell list, which its UEs use to know what frequencies and cells to measure and report upon in their scheduled measuring opportunities. These neighbor cell measurement reports aid the serving cell in handling UE mobility.
Current LTE specifications provide for a UE mobility state estimation (MSE) which is based on the number of cell reselections (for a UE in the idle mode) or handovers (for a UE in the connected mode) the UE has undergone over a predefined period of time. This ratio serves as a proxy for the UE's relative speed. For idle mode UEs having medium and high mobility states, the UE will add an offset to the signaled cell reselection margin (the parameter Qhyst which is a hysteresis value) and scales the value for Treselection by a factor. Treselection is a time value signaled also in system information against which a UE will test the received signal strength (or power) of a best neighbor cell against that of its serving cell; if the received signal strength/power from the neighbor cell exceeds that from the serving cell for the duration Treselection the UE is to re-select to that neighbor cell. The connected mode UE will scale its time-to-trigger by a factor. Time to trigger is a time window over which the UE averaged measurements have to fulfill a given event prior to the UE sending a given measurement report for that event (for an event triggered measurement report). These scaling factors are parameters signaled by the network and are used at least in part to optimize mobility for fast moving UEs.
These factors were not designed with HetNets in mind, but rather for the case where the UE in FIG. 1 would see only macro neighbor cells 20, 22; at that time pico neighbor cells 16, 18 were not a part of the radio environment. See for example 3GPP TS 36.304 and 36.331. So the above parameters and procedures for using them intended that the faster moving UEs should hand over faster than slower moving UEs, so as to avoid late handovers and radio link failures due to a long delay by setting those parameters from the baseline of a slow-moving UE). The mobility state estimation noted above can be used for this purpose. But the conventional macro cell processes are not optimized for HetNet deployments as will be detailed below.
What is needed in the art is a way to account for pico cells and similar in a HetNet radio environment when considering UE mobility.